1. Field of the Invention
This invention relates generally to deposition of dispersed particles on substrates and the resulting compositions.
2. Background
A supercritical fluid (SCF) is a substance above its critical temperature (Tc) and pressure (Pc). SCFs have been used as solvents in numerous applications, such as the polymerization of ethylene (Odian, G. G. Principles of Polymerization, John Wiley & Sons (1991)), decaffeination of coffee (McHugh, M. and Krukonis, V. Supercritical Fluid Extraction, 2nd ed., Butterworth-Heinemann, Newton (1994)), for organic chemical reactions (Kaupp, G., “Reactions in Supercritical Carbon Dioxide,” Angewandte Chemie, 33, 1452–1455 (1994); Johnson, K. P., “Safer Solutions for Chemists,” Nature, 368, 187–188 (1994)), and nanocomposite synthesis (Watkins, J. J. and McCarthy, T., “Polymer/Metal Nanocomposites in Supercritical CO2,” Chemistry of Materials, 7, 1991 (1995); Watkins, J. J., Chemistry in Supercritical Fluid-Swollen Polymers: Direct Synthesis of Polymer/Polymer and Polymer/Metal Composites, Ph.D. in Polymer Science and Engineering, University of Massachusetts at Amherst (1997); Watkins, J. J. and McCarthy, T. J., “Polymerization of Styrene in Supercritical CO2-Swollen Poly(chlorotrifluoroethylene),” Macromolecules, 28, 4067–4074 (1995); Cansell, F., Cheavlier, B., Demourgues, A., Etourneau, J., Even, C., Garrabos, Y., Pessy, V., Petit, S., Tressaud, A. and Weil, F., “Supercritical Fluid Processing: A New Route for Material Synthesis,” Journal of Materials Chemistry, 9, pp. 67–75 (1999)). The applications of SCFs arise due to several of its characteristics, such as a wide range of solvent strengths and densities that can be adjusted by tuning the pressure and/or temperature.
A number of SCFs have been used as solvents and co-solvents for the production of nanoparticles and for micron-sized particles (Johnson, K. P., “Safer Solutions for Chemists,” Nature, 1994, 368, 187–188; Cansell, F., et al., J. of Mat'l Chem., 1999, 9, pp. 67–75). Particle design is becoming a very important application for SCFs, especially in the pharmaceutical industry (Park, Y., Curtis, C. W., and Roberts, C. B., “Formation of Nylon Particles and Fibers Using Precipitation with a Compressed Antisolvent,” Industrial & Eng. Chem. Res., 2002, 41, 1504–1510).
Rapid expansion of supercritical solutions is a process in which the material of interest is dissolved in a SCF and rapidly depressurized through a nozzle, causing an extremely rapid nucleation of the product (Park, Y., et al., 2002, 41, 1504–1510). Another common method for the production of micron-sized particles is the formation of particles from gas-saturated solutions (Park, Y., et al., 2002, 41, 1504–1510). This process consists of dissolving into a supercritical fluid a liquid material or a solution of the material. The mixture is then passed through a nozzle causing the formation of liquid droplets and the growth of particles. These methods can allow for control of the crystal structure and the size of the particle, which is important as the crystal structure can have a large impact on biological functionality (Kordikowski, A., York, P., and Latham, D., “Resolution of Ephedrine in Supercritical CO2: A Novel Technique for the Separation of Chiral Drugs,” J. Pharm. Sci., 1999, 88, 786; Park, Y., et al., 2002, 41, 1504–1510). These methods for precipitating particles within SCFs have been extended into polymers. In one example, micrometer sized particles and fibers of nylon 6/6 were produced by expanding polymer solutions into SC—CO2.
U.S. Pat. No. 4,737,384 to Murthy et al., is directed to “a process for depositing a thin metal or polymer coating onto a substrate. More particularly, the process of this invention comprises the steps of: exposing a substrate at supercritical temperatures and pressures to a solution comprising a metal or polymer dissolved in water or a non-polar organic solvent, said metal or polymer being substantially insoluble in said solvent under sub-critical conditions and being substantially soluble in said solvent under super critical conditions; and, reducing the pressure, or temperature and pressure to sub-critical values, thereby depositing a thin coating of said metal or polymer on said substrate.” See Summary of the Invention, col. 2, lines 11–24.
U.S. Pat. No. 5,789,027 to Watkins et al., is directed to methods for “depositing a film of material on the surface of a substrate by i) dissolving a precursor of the material into a supercritical or near-supercritical solvent to form a supercritical or near-supercritical solution; ii) exposing the substrate to the solution, under conditions at which the precursor is stable in the solution; and iii) mixing a reaction reagent into the solution under conditions that initiate a chemical reaction involving the precursor, thereby depositing the material onto the solid substrate, while maintaining supercritical or near-supercritical conditions. The invention also includes similar methods for depositing material particles into porous solids, and films of materials on substrates or porous solids having material particles deposited in them.” See Abstract.
U.S. Pat. No. 6,132,491 to Wai et al., is directed to “a method for dissociating metal-ligand complexes in a supercritical fluid by treating the metal-ligand complex with heat and/or reducing or oxidizing agents is described. Once the metal-ligand complex is dissociated, the resulting metal and/or metal oxide form fine particles of substantially uniform size. In preferred embodiments, the solvent is supercritical carbon dioxide and the ligand is a β-diketone such as hexafluoroacetylacetone or dibutyldiacetate. In other preferred embodiments, the metals in the metal-ligand complex are copper, silver, gold, tungsten, titanium, tantalum, tin, or mixtures thereof. In preferred embodiments, the reducing agent is hydrogen. The method provides an efficient process for dissociating metal-ligand complexes and produces easily-collected metal particles free from hydrocarbon solvent impurities. The ligand and the supercritical fluid can be regenerated to provide an economic, efficient process.” See Abstract.
U.S. Pat. No. 6,592,938 B1 to Pessey et al., is directed to “a method for coating particles thus obtained. According to the inventive method, the particles that are to be coated and at least one organo-metallic complex precursor of the coating material are brought into contact with each other in a liquid containing one or several solvents, whereby said particles are maintained in a dispersion in the liquid which is subjected to temperature conditions and supercritical pressure or slightly sub-critical pressure conditions; the precursor of the coating material is transformed in such a way that it is deposited onto the particles, whereupon the liquid is placed in temperature and pressure conditions so that it can eliminate the solvent in a gaseous state. The invention can be used to coat nanometric particles in particular.” See Abstract.
The prior art does not disclose the particular advantageous steps or features of the present invention. Among other reasons, the prior art does not disclose the combination of a reducing reaction, particulate metal being formed rather than a metal film, a particulate substrate, and/or venting prior to reducing.
For the above reasons, the ability to adequately disperse particles on particulate substrates has not yet been met.